1. Field of the Invention
The present invention relates to accelerometers generally and, more particularly, but not by way of limitation, to a novel method and means for cancellation of accelerometer sensor noise due to dynamic pitch motion of the platform on which the sensor is mounted.
2. Background Art.
A need has long existed for a method and means to reduce or eliminate flow noise and mechanically induced noise in directional hydrophones. This noise is predominant near the low end of the frequency range of interest (near 10 Hz), and generally increases in severity as the flow increases. The noise is due, in part, to mechanical vibrations induced by flow past various components of the system and, in part, to motion induced into the system due to surface waves. The directional sensor may be regarded as a very sensitive accelerometer which responds to these mechanical vibrations and perturbations. These vibrations are coupled to the sensor through the support cables and suspension components of the buoy system. The present invention relates to means by which the sensor's sensitivity to these motion induced vibrations can be reduced; however, it will be apparent to those skilled in the art that the invention may be readily applied to other types of accelerometer based sensors.
Many direction hydrophones, microphones or accelerometer sensors, operate basically as accelerometers whose main axis of sensitivity is in the horizontal plane. Such devices not only respond to the horizontal acceleration of the mounting platform due to the acoustic driving force (or acceleration of the platform on which the sensor is mounted), but also to other forces which accelerate the body in translational or rotational motion. Such an accelerometer based sensor will also respond to the component of gravity which acts along the sensitive axis of the device.
If the sensor platform is periodically tilted about a horizontal axis normal to the sensitive axis of the horizontal accelerometer, then the gravitational force component in the direction of the sensitive axis of the sensor will cause a periodic output from the sensor indistinguishable from the output caused by a periodic acceleration or from the output due to an acoustic excitation. This gravity induced output is one form of undesirable motion induced noise caused in this case by pitch motion about a horizontal axis.
There is also a component of acceleration in the output of the sensor caused by rotational accelerational effects as the sensor platform periodically tilts about a horizontal axis, the same motion described above. The amplitude of this acceleration component is given by the angular acceleration of the platform times the distance from the axis of rotation to the reaction mass of the sensor.
Accordingly, it is a principal object of the present invention to provide a method and means to reduce noise experienced by accelerometer sensors.
It is another object of the invention to provide such means that is easily and economically constructed.